Color half-tone printing devices deposit marking agents, such as ink or toner, having a small number of colors onto an image receiving surface to form a printed image. The marking agent is deposited in sufficiently small quantities to enable the human eye to perceive a large number of potential colors from different combinations of the marking agents. For example, many printers use cyan, magenta, yellow, and black (CMYK) marking agents to form multi-color printed images. For example, in an inkjet printer, different groups of inkjets eject CMYK inks in predetermined half-tone drop patterns to form the color images, and in a xerographic printer the CMYK toners are deposited onto a photoconductive drum or belt and are subsequently transferred to a print medium.
One type of half-tone color printer is a drop on demand inkjet printer. Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops from a plurality of inkjets, which are arranged in one or more printheads, onto an image receiving surface. In a direct inkjet printer, the printheads eject ink drops directly onto the surface of a print medium such as a paper sheet or a continuous paper web. In an indirect inkjet printer, the printheads eject ink drops onto the surface of an intermediate image receiving member such as a rotating imaging drum or belt. During printing, the printheads and the image receiving surface move relative to one other and the inkjets eject ink drops at appropriate times to form an ink image on the image receiving surface. A controller in the printer generates electrical signals, also known as firing signals, at predetermined times to activate individual inkjets in the printer. The ink ejected from the inkjets can be liquid ink, such as aqueous, solvent, oil based, UV curable ink or the like, which is stored in containers installed in the printer. Alternatively, some inkjet printers use phase change inks that are loaded in a solid form and delivered to a melting device. The melting device heats and melts the solid phase change ink to a liquid form that is supplied to a printhead for printing as liquid drops onto the image receiving surface.
During the operational life of these printers, inkjets in one or more printheads may become unable to eject ink or to eject ink drops of an appropriate mass in response to a firing signal. These inkjets are described as defective or inoperable inkjets in this document. When an inkjet becomes inoperable, any printed image that requires the use of the inoperable inkjet is formed with an image defect. In printer embodiments where an image receiving surface, such as a sheet or elongated roll of paper, moves past an array of printheads in a print zone, the image defect is often visible as a narrow unprinted line or “light streak” that extends through the printed image. The light streak appears in locations of the image where the inoperable inkjet would have ejected ink drops if the inoperable inkjet were functioning correctly.
Existing compensation methods enable a printer to form printed images that reduce or eliminate the visible impact of inoperable inkjets on the printed images. For example, some printers operate in a default mode where the relative coverage of the printed ink that forms printed images is set to a fraction of the maximum coverage that the printer is capable of producing. For example, a printer receives image data corresponding to a printed image and reduces the intensity of the image data by 20% to form the printed image using an 80% intensity. During operation the printer increases the intensity level for portions of the printed image that are located near an inoperable inkjet when an inoperable inkjet is detected. The increased coverage of ink around the light streak corresponding to the inoperable inkjet and the spreading of the ink on the printed paper reduces the perceptibility of the light streak. The reduced ink coverage in the default print mode provides a margin for the printer to increase the ink coverage in selected regions of the printed image to compensate for one or more inoperable inkjets.
One drawback to printing images with a reduced intensity level is that the printed quality of some colors is negatively impacted due to the reduced coverage of ink in the printed image. For example, many color printers form images using a small number of ink colors, such as CMYK inks. Different combinations of the ink colors that are printed as separate drops in close proximity to each other or that merge together in the printed image form a wide range of visible colors to the human eye. To the human eye, however, the reduced intensity of some colors in the printing process results in a much more perceptible impact to the quality of the printed image for some colors in comparison to other colors. For example, in a CMYK printer, the modified printing process affects the image quality of the color red to a greater degree than many other printed colors. In some instances, different shades of red appear to be orange due to the reduction in the intensity of the printed image. Thus, the impact on the quality of the printed images is greatly affected by the color content of the images, which can lead to reduced image quality. Consequently, improvements to the process for printing images that improve the quality of the color reproduction in the printed images would be beneficial.